1

Geotechnical Failure Investigation of a Reinforced Soil Wall and Remedial Work Design

Tan, Yean-Chin; Kowng, Yan-Wen & Tan, Su-Kwong G&P Geotechnics Sdn Bhd, Wisma G&P, 39-5 Jalan 3/146, Bandar Tasik Selatan, Kuala Lumpur, Malaysia. gnp-geo@gnpgroup.com.my

This paper presents a case study of a displaced reinforced soil (RS) wall which is located on a fill slope with a bulging wall and open-ing of gaps within the wall. Subsurface Investigation (S.I.) had been carried out using exploratory boreholes, mackintosh probes and laboratory tests to determine the relevant engineering properties of the subsoil for subsequent slope stability analyses and investigation. The investigation revealed that the failure is mainly attributed to inappropriate foundation design. This paper describes the site condi-tions, including foundation design and the details of the geotechnical investigation of the distressed wall. Following the geotechnical investigation and analyses, remedial works have been proposed and successfully constructed which include reconstructing part of the wall to be supported by slab with spun piles and the use of lightweight materials of Expanded Polystyrene (EPS).

1 INTRODUCTION

The platform of a petrol station was constructed with a rein-forced soil (RS) wall of up to a height of 7.5m. The site is lo-cated in Kuala Lumpur. The RS wall is located on top of a fill slope and is supported by reinforced concrete (RC) square piles. The movements of foundation and progressive wall bulging had led to the opening of gaps within the wall and ag-gravated the stability of the wall. This paper presents a geo-technical investigation carried out by the Authors to find out the causes of the failure. Appropriate remedial measures were subsequently proposed based on the findings of the geotechni-cal investigation works.

2 TOPOGRAPHICAL AND GEOLOGICAL CONDITIONS

Generally the original ground surface at site varies from about RL 87m to RL 109m before the construction of the platform at RL 102.83m. The geological map of Selangor Darul Ehsan, compiled by the Geological Survey Department, indicates that the project site is underlain by Kuala Lumpur Granite. The texture and composition of granitic rock generally ranges from coarse to very coarse-grained. The subsoils based on grain size generally consists of clayey sandy SILT with some layers of silty sandy CLAY and silty sandy GRAVEL which is typi-cal of granitic residual soils.

3 ORIGINAL REINFORCED SOIL WALL AND FOUNDATION DESIGN THAT FAILED

The original RS wall foundation consists of 150mm x 150mm square Reinforced Concrete (RC) piles at 1m c/c spacing driven to a penetration length of 9m. A 350mm thick RC slab is subsequently cast on the piles as the foundation of the RS wall. The piling layout of the original RS wall design is shown in Figure 1.

The 7.5m high RS wall design consists of well compacted granular backfill and galvanised steel reinforcing strips of highly adherent type (5mm thick x 45mm width) with length of 5.8m for the top half height of the wall and length of 5.0m for the lower half height of the wall. Typical cross section of RS wall design is shown in Figure 2.

Figure 1 Original Piling Layout of RS Wall

Figure 2 Typical Section of Reinforced Soil Wall

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4 SITE CONDITIONS AND OBSERVATIONS

The RS wall panel showed visible signs of displacement, bulging and opening of gaps between the wall panels at the south-western corner of the wall as shown in Figure 3. This had caused backfilled material to be washed away when rain, inducing voids behind the RS wall as shown in Figure 4 that further aggravated the internal stability of the wall.

5 SUBSURFACE INVESTIGATION

There were two stages of Subsurface Investigation (S.I) works being carried out at the site. The first stage of S.I. works (borehole designation with ABH) were carried out prior to earthwork by the design consultant. The S.I works consist of two (2) boreholes and ten (10) Mackintosh Probes (AMP). However, the results obtained from the first stage of S.I works were limited and insufficient for detailed investigation and remedial work design. In addition, the boreholes are located at quite a distance apart from the location of the wall. There-fore, some of the important soil parameters could not be estab-lished due to insufficient data. A second stage of S.I. works (borehole designation with BH) was subsequently proposed by the Authors to assist in the investigation. The second stage S.I. works consist of one (1) borehole and eighteen (18)

Mackintosh Probes (MP). The locations of field tests for both stages of S.I are shown in Figure 5.

Figure 6 shows the interpreted subsoil profiles for both stages of S.I. works. Generally, borelogs of ABH-1 and ABH-2 indi-cate that the original subsoil consists of sandy SILT and silty GRAVELS. Based on the subsurface investigation carried out, the overburden soils in borehole BH-1 generally consist of clayey SAND at the top 5 m, which are the fill materials and underlain by the original residual subsoil. The depth of fill materials is consistent with the original topography plan, which shows that the original ground level is about 6m below the existing profile. The clayey SAND and sandy CLAY are generally very loose to loose and soft to medium stiff respec-tively with SPT ‘N’ values ranging between 3 and 11 up to a depth of approximately 8 m followed by slightly weathered granitic bedrock. MP tests from the second stage of S.I. works were carried out along the slope as shown in Figure 5. Figures 7 and 8 show the MP profiles, which are generally consistent with the fill thickness.

Figure 3 Side View of Wall Bulging

Figure 4 Voids Behind Wall Due to Backfill Materials Being Washed Away

Figure 5 First Stage and Second Stage of S.I Works

Figure 6 Interpreted Subsoil Profile from Both Stages of S.I. Works

Bulging and Gap Opening

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5.1 Laboratory Test Results

A series of laboratory tests were carried out on the samples obtained from the second stage of subsurface investigation works, they are: 1. Atterberg limits – see Figure 9 2. Particle size distribution 3. Single stage direct shear box test for wall granular backfill

– see Figure 10 4. Isotropically consolidated undrained triaxial test (C.I.U)

for in-situ subsoil – see Figure 11 The results of the above laboratory tests are summarised in Table 1.

Table 1: Summary of Laboratory Test Results Type of Laboratory Test

Results

Atterberg Limits & Particle Size Distri-bution

Samples collected from residual sub-soil are mostly clayey sand of low plasticity

Single Stage Direct Shear Box Test

c = 0 kPa and φ = 36° for granular backfill material of the RS wall

C.I.U c = 3 kPa and φ = 33° for residual subsoil

Figure 7 Mackintosh Probe Profiles for Section 1-1

Figure 8 Mackintosh Probe Profiles for Section 3-3

Figure 9 Atterberg Limit

Figure 10 Single Stage Direct Shear Box Test Results for

Granular Backfill Material of RS Wall

0 10 20 30 40 50 60 70 80 90 100 110 120

0

10

20

30

40

50

60

70

80

Pla

stic

ity In

dex

I p

Inorganic Clays of Medium Plasticity

Inorganic Clays of High Plasticity

Inorganic Silts of High PlasticityInorganic Silts of Medium

Plasticity

Inorganic Clays of Low Plasticity

Inorganic Silts of Low Plasticity

CohesionlessSoils

wl=35

wl=50 Line "A" Ip=0.73 (wl-20)

Liquid Limit wl

wl=70

wl=90

Inorganic Clays of Very HighPlasticity

Inorganic Silts of Very High Plasticity

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6 GEOTECHNICAL INVESTIGATION INTO THE FAILURE OF RS WALL

In order to determine the causes of RS wall bulging and dis-placement, the following analyses as shown in Table 2 were carried out based on the interpreted subsoil parameters and backfill material properties obtained from laboratory tests:-

Table 2: Type of Analyses Carried Out

The following subsection describes in detail the analyses car-ried out to investigate the causes of distress.

6.1 Global Stability Analyses

6.1.1 Slip Failure Analysis

Independent slope stability analyses were carried out to check for different modes of failures namely circular and irregular shaped failure surfaces using the Modified Bishop’s and Spencer’s Methods respectively. Therefore, all possible slip failure surfaces have been checked and the most critical Fac-tor of Safety (FOS) for the slopes is computed. In the slope stability analyses, the following conditions were analysed: Case 1: RS wall with the presence of 150mm x 150mm rein-forced concrete (RC) piles as per the original design by C&S consultant (assuming the piles were not displaced)

Case 2: RS wall without the piles to simulate the FOS in the initial construction before installation of piles and if the small piles had displaced Case 3: Local fill slope stability in front of RS wall The acceptability of a slope’s stability is based on its ability to achieve an adequate Factor of Safety (FOS) against slope fail-ure. An appropriate FOS against the failure of a slope is de-pendent upon the extent to which that failure could potentially result in the loss of life. It is also dependent upon the degree of economic loss that would result if a failure does occur. The assignment of an appropriate FOS for various types of slopes would be done in accordance to the Hong Kong Geotechnical Engineering Office’s (GEO) “Geotechnical Manual for Slopes”. Factor of Safety of 1.4 is recommended in the stabil-ity analyses of the slopes. In addition, the slopes should achieve a factor of safety of 1.1 for the condition of “worst predicted groundwater level”. Based on the results of the analyses as shown in Table 3, the RS wall displacement is not likely to be induced by local in-stability of fill slope in front of the wall. Based on the slip failure analyses results, it can be deduced that the pile dis-placement and lateral resistance are possible critical factors in-fluencing the global stability. As deformation could not be analysed in the slip failure analyses using the limit equilib-rium method, finite element method (FEM) analyses are re-quired to assess the pile displacement and lateral resistance.

Table 3: Stability Analyses Results

Long Term Factor of Safety Case

Section Modified Bishop Method

(Circular Failure)

Spencer’s Method (Non-

Circular Failure)

With Piles 1.58 (>1.4) 1.69 (>1.4)

Case 1 With Piles (worst case of WL)

1.26 (>1.1) 1.37 (≥1.1)

Without Piles 1.25 (<1.4) Not acceptable

1.36 (<1.4) Not acceptable

Case 2 Without Piles (worst case of WL)

1.20 (> 1.1) 1.25 (>1.1)

Local Stability of 1V:1.5H Fill Slope

1.37 (<1.4) Not acceptable

1.44(>1.4)

Case 3 Local Stability of 1V:1.5H Fill Slope

(worst case of WL)

1.37 (>1.1) 1.44 (>1.1)

Figure 11 C.I.U Test Results for Residual Subsoil

Type of Analysis Details

Global Stability of the Wall and Slope

• Slip Failure Analysis • Overturning and Bearing

Capacity Check • Sliding Failure Check

Internal Stability of RS Wall • Rupture • Adherence

Structural and Geotechnical Capacity Check of the Piled Foundation

• Pile Axial Capacity Check • Pile Lateral Resistance

Check • RC Base Slab Check

Degree of Compaction of Fill Slope Materials

• Check on Adequacy of De-gree of Compaction

Interpreted Soil Strength Parameters (C.I.U. Tests)

0

50

100

150

200

250

0 50 100 150 200 250 300 350

S = (σ1' + σ3') / 2

T =

( σ1'

- σ

3') /

2

Lower Upper AdoptedBH1/UD1/3-3.9 BH1/UD1/3-3.9 BH1/UD1/3-3.9BH1/UD2/6-6.9 BH1/UD2/6-6.9 BH1/UD2/6-6.9

c = 3 kPa φ = 33°

5

As shown in Table 3, the FOS for Case 1 is adequate in both abovementioned groundwater conditions. It indicates the global stability of the slope is adequate if the RC piles were not displaced significantly and could provide adequate lateral resistance against slope movement. However, if the piles have displaced or damaged due to slope movement, the global sta-bility would be inadequate, as demonstrated by Case 2. Al-though the local stability of the slopes is slightly inadequate (FOS=1.37 < 1.4) as shown in Case 3, it is only marginal. In view of this, the RS wall displacement is not likely to be in-duced by local instability of fill slope in front of the wall, but rather due to global slope instability with loading from RS wall when the small piles had been displaced, as shown in Case 2. The original configuration of RS wall supported by RC piles has been analysed using a two-dimensional finite element method programme. Figure 12 shows the graphical printouts of the FEM analyses results (the deformation is plotted in ex-aggerated scale). As shown in Figure 12, the RC piles beneath the RS wall have bent and displaced significantly, and there-fore part of the RS wall supported by piles also displaced to-gether. The estimated pile displacement ranges from about 150mm to 170mm with induced maximum bending moment and shear force to the pile of 36 kNm and 64 kN respectively. The maximum mobilised bending moment and shear force in-duced onto the 350 mm thick slab is 146 kNm and 192 kN re-spectively. The comparison of the induced and ultimate bend-ing moment and shear force of piles and slab are elaborated in Section 6.3.2 and 6.3.3 respectively.

6.1.2 Overturning and Bearing Capacity Check

The long term bearing capacity of the wall is governed by the pile foundation supporting it. As for overturning, the inde-pendent analyses carried out indicate that the wall is stable against overturning with factor of safety against failure of greater than 2.0.

6.1.3 Sliding Failure Check

The sliding resistance is provided by the soil-to-soil contact between the reinforced soil wall fill and the slab supporting it. The analysis indicates that the sliding resistance is adequate against the lateral forces imposed on the RS wall. However, it is important to note that the lateral forces have been trans-ferred to the piles which should have been designed to cater for it.

6.2 Internal Stability Analyses

Internal stability analyses were carried out to check the inter-nal stability of the RS wall in terms of rupture, adherence and wedge stability in accordance to BS 8006:1995. Coherent Gravity Method is adopted as recommended in Clause 6.3 of the BS 8006:1995. As required, three load cases of load com-bination with different partial load factors are considered in the design. Table 4 shows the load combinations used for the checking of the design of the reinforced soil wall. In addition, the effect of groundwater level on the internal stability is also investigated using the method proposed by Tan & Khoo (2006). Combination A considers the maximum values of all loads and therefore normally generates the maximum reinforcement tension and foundation bearing pressure. It may also deter-mine the reinforcement requirements to satisfy pull-out resis-tance although pull-out resistance is usually governed by combination B. Combination B considers the maximum overturning loads to-gether with minimum self mass of structure and superimposed traffic load. This combination normally dictates the rein-forcement requirements for pull-out resistance and is normally the worst case for sliding along the base. Combination C considers dead loads only without partial load factors. This combination is used to determine foundation set-tlements as well as generating reinforcement tensions for checking the serviceability limit state. Table 4: Partial Load Factors for Load Combinations associ-ated with Walls (from BS8006:1995)

Load Combination

Effects A B C

Mass of reinforced soil body 1.5 1.0 1.0

Mass of backfill on top of RS Wall 1.5 1.0 1.0

Earth Pressure behind struc-ture 1.5 1.5 1.0

Traffic Load:

(a) on reinforced soil block 1.5 0.0 0.0

(b) behind reinforced soil block 1.5 1.5 0.0

6.2.1 Rupture

Rupture is the tensile capacity of the reinforcement to resist the tensile forces generated from the lateral earth pressure and the external loads. This is specified under BS 8006:1995 clause 6.6.5.2.5. Independent analyses carried out indicate that the reinforcement is adequate against rupture failure.

6.2.2 Adherence

Adherence is the resistance of the reinforcing fill against local sliding of the reinforcement from the tensile forces generated.

Figure 12 FEM Analysis – Displaced RC Piles

6

The adherence check is specified under BS8006:1995 clause 6.6.5.2.4. Based on the design calculations provided, the original design methodology of RS wall is in accordance to BS8006:1995. However, the values for the coefficient of fric-tion (μ) used are following the French Ministry of Transport’s (FMT) recommendations for Reinforced Earth Structures (FMTRRES) instead of BS8006:1995. The FMT’s recommendations use the parameter f* which is similar to BS8006:1995’s μ parameter. The value used varies with depth of the reinforced soil wall as follows (Cl. 2.3.3.1, FMTRRES):

f * = fo

* (1 – z/zo) + tan (φ’). z/zo for Z ≤ Zo = 6m.

f * = tan (φ’) for Z > Zo

Where the minimum fo* value is taken as 1.50 in absence of

accurate measurements as recommended by FMT. British Standard for Reinforced Soil, BS8006:1995 states that the coefficient of friction is derived from μ = tan δ’. However, BS8006:1995 does not state the value of δ’ and in consistent with the standards, the value of δ’ is obtained from British Code of Practice for Earth Retaining Structures (BS8002:1994). Clause 3.2.6 of BS8002:1994 states that for design values, the values of parameter δ’ can be adopted as approximately two-thirds of the peak angle of friction of fill materials in the reinforced soil (2/3*φ’). Using BS8002:1994’s design values for δ’ results in coeffi-cient of friction value of 0.45 (for a soil peak angle of friction of 36°. However, using FMT’s recommendations, the f* pa-rameter values varies from maximum 1.5 at the top of the RS wall to tan φ’ (which is 0.73) from the depth below 6 m. Therefore, the FMT’s recommendations of μ values are higher than the British Standards by 62%. The design of RS wall us-ing FMT recommendations is more optimistic than design us-ing British Standards. Figure 13 presents the results of the tensile force and adher-ence (calculated in accordance to BS8006:1995 and FMT) for all cases. As can be seen from Figure 13, the tensile force in-duced is higher than adherence. The non-compliance on ad-herence failure according to BS8006:1995, is due to insuffi-cient reinforcement strip length provided there is no change to the strip width. The reinforcement for adherence is marginally adequate at the top 3m based on FMT’s recommendations. It can be seen that the adherence reduces at the depth of ap-proximately 3.5m for both BS8006 and FMT. This is attrib-uted to the reduction in the length of steel reinforcing strips as explained in Section 3. In addition, the level of compaction of the reinforced fill ma-terials during construction (if inadequate) will also affect the adherence.

6.2.3 Effect of Groundwater Level on Internal Stability

In order to investigate the effect of groundwater tables on the internal stability of RS wall, groundwater table of 1/3 and 2/3 of RS wall height are assumed in the analyses according to BS8006:1995. The tensile force does not significantly increase with the presence of groundwater table (GWL) as can be seen from Figure 14.

The effect of groundwater table on the adherence is also in-vestigated. The adherence (in accordance to BS8006) with the presence of the groundwater table (at 1/3H and 2/3H) is ex-pectedly lower than that without the presence of the ground-water table as illustrated in Figure 15.

Figure 13 Tensile Force Induced and Adherence Profile With Depth

Figure 14 Acting Tensile Force Profile with Depth (Case A, B, C)

GWL @ 2/3H

GWL @ 1/3H

0 10 20 30 40 50 60 70Tensile Force / Adherence (kN)

8

7

6

5

4

3

2

1

0

Dep

th (m

)

0 10 20 30 40 50 60 70

8

7

6

5

4

3

2

1

0Adherence (FMT)Adherence (BS8006)Tensile Force - Case CTensile Force - Case BTensile Force - Case A

0 10 20 30 40 50 60 70 80Tensile Force (kN)

8

7

6

5

4

3

2

1

0

Dep

th (m

)

0 10 20 30 40 50 60 70 80

8

7

6

5

4

3

2

1

0No water effect - Case ANo water effect - Case BNo water effect - Case CWater@1/3H - Case AWater@1/3H - Case BWater@1/3H - Case CWater@2/3H - Case AWater@2/3H - Case BWater@2/3H - Case C

7

When comparing the adherence and tensile force induced with the presence of groundwater table (Figure 16), it was found that the adherence is still inadequate. The adherence calcu-lated using FMT is also included for comparison. In both methods of calculation of adherence, the tensile force exceeds the adherence with and without the presence of groundwater table indicating inadequacy of design length.

6.3 Structural and Capacity Check of Piled Foundation

6.3.1 Pile Axial Capacity Check

The geotechnical capacity of the piles is estimated by using the borehole information. The calculations show that the geo-technical axial capacity of the piles is adequate for the pro-posed pile working load of 200kN. High strain dynamic load tests were carried out on two piles and the results indicate that the tested piles had achieved an axial static resistance of more than twice the working load at the time of testing before plac-

ing of the wall. Therefore, the geotechnical axial compression capacity of the piles is adequate.

6.3.2 Pile Lateral and Bending Resistance Check

Shear force and bending moment mobilised in the RC piles are extracted from the FEM analyses. As mentioned in Section 5.1.1, the maximum bending moment and shear force induced on the RC piles are 36 kNm and 64 kN respectively. The steel reinforcement details of the 150mm x 150mm RC pile that are commonly available in the local market using 4Y10 with link of φ5.5@69mm are adopted for computation of moment and shear resistances. The ultimate bending moment and shear force capacity of 150mm x 150mm pile are 10 kNm and 31 kN respectively and are found to be grossly inadequate. As a result of inadequate lateral resistance, the RC piles have struc-turally failed under bending and shear. Since the piles are embedded within the 4m thick soil layer, these piles maybe subjected to axial and lateral loads which are induced by these soil movements. Tschebotarioff (1973) presented design method to predict maximum induced bend-ing moment for such scenario. As such, additional analysis us-ing Tschebotarioff’s method was also carried out to check the adequacy of the piles to resist such loads. The estimation of the lateral force and bending moment is based on the assump-tion of fixed pile head condition. The results show that the es-timated lateral force and maximum bending moment are 28 kN and 40 kNm respectively which are relatively consistent with the findings from FEM analyses. However, it shall be noted that this method only provides an estimate of the pile bending moment and does not consider either the lateral de-flection or the axial response.

6.3.3 RC Base Slab Check

Independent analysis of the adequacy of the RC base slab was carried out based on the information from drawings prepared by the original consultant. Based on the drawings, BRC mesh A10 was provided at the top and bottom of the base slab of 350mm thick. As mentioned in section 6.1.1, from the FEM analyses, the maximum bending moment and the shear force induced onto the RC slab are about 146 kNm/m and 192 kN/m respectively. Based on BS8110:1985, the ultimate moment and shear resistances of the slab are 55 kNm/m and 102 kN/m respectively. Therefore, the moment and shear resistances of the slab are grossly inadequate.

6.4 Compaction of Fill Slope Material

Compaction tests were carried out on the fill materials during filling of the platform and slopes and prior to the construction of the RS wall. The information on the locations of the tested samples was not made available at the time of investigation. Figure 17 shows the compaction test results which indicate that the degree of compaction of tested samples ranges from 82% to 99%. In other words, 56% of the tested samples have a degree of compaction less than 95%, indicating some local-ised areas with inadequate compaction. This can be observed from some of the Mackintosh Probes results (Figures 7 to 8), in which soil consistency increases and decreases alternately with depth at some MP locations.

Figure 15 Adherence Profile With Depth (BS8006)

Figure 16 Adherence and Tensile Force Profile with Depth (With Water Effect)

0 10 20 30 40 50 60 70 80Tensile Force / Adherence (kN)

8

7

6

5

4

3

2

1

0

Dep

th (m

)

0 10 20 30 40 50 60 70 80

8

7

6

5

4

3

2

1

0Adherence (FMT)Adherence (BS8006)Tensile Force - Case CTensile Force - Case BTensile Force - Case A

GWL @ 1/3H

GWL @ 2/3H

GWL @ 1/3H

0 10 20 30 40 50Adherence (kN)

8

7

6

5

4

3

2

1

0

Dep

th (m

)

0 10 20 30 40 50

8

7

6

5

4

3

2

1

0No water effect - Case A,B,CWater@1/3H - Case A,B,CWater@2/3H - Case A,B,C

8

6.5 Findings

Based on the above findings, the causes of RS wall displace-ment and bulging are as follows: a) Main factors: Foundation instability mainly due to inade-quate pile lateral resistance against lateral movement and in-adequate shear and moment resistances of the RC piles and slab b) Inadequate FOS for RS wall reinforcement strip adherence and inadequate fill compaction also contributed to some bulg-ing of the wall The inadequacy of the abovementioned main factors is con-firmed via visible cracks observed on the slab upon exposing the base of the RS wall (as shown in Figure 18) for the reme-dial work.

In addition, due to the opening of the gap and bulging of the RS wall caused by the factors described above, the fill materi-als (granular materials) in the RS wall have dropped and washed out through the openings. The condition deteriorated more rapidly during rainy season. Due to the loss of materi-als, the internal stability of the wall was badly affected, induc-ing further widening of gap and bulging of the wall. The proc-esses repeated until a large void was formed in the wall (as shown in Figure 4) and depression at the top of the wall.

7 PROPOSED REMEDIAL WORK

The proposed remedial work includes the following construc-tion sequences:- a) Installation of sheet pile b) Excavation of backfill material to the base of the rein-forced soil wall c) Hacking the base slab and drive φ400mm spun piles (Grade 80, Class B) filled with reinforced concrete plug d) Reinstatement of Reinforced Soil (RS) wall and partial re-placement of backfill material with Expanded Polystyrene (EPS) Since the convenient store is located near the slope, temporary sheet piles were necessary to minimise ground movement at the convenient store during the adjacent excavation.

The spun piles were closely spaced and filled with reinforced concrete plug to provide adequate lateral resistance and the piles were driven into the competent hard layer. As such, it is not necessary to adopt raking piles to provide the lateral resis-tance.

Longer wall reinforcement length was also used to achieve the required factor of safety of the wall against adherence failure while Expanded Polystyrene (EPS) was adopted to reduce the vertical overburden pressure at the wall section where re-placement of wall reinforcements could not be done due to site constraints. Figures 19 to 24 illustrate the details and se-quence of the proposed remedial works.

Figure 17 Compaction Test Results

Figure 18 Slab Cracks Observed

Figure 19 Installation of Sheet Pile

Slab

Bulk Density vs Degree of Compaction

0

0.5

1

1.5

2

2.5

80 82 84 86 88 90 92 94 96 98 100Degree of Compaction(%)

Bulk

Den

sity

(Mg/

m3)

Test A Test B

9

Figure 23 Section A-A

Figure 24 Section B-B

7.1 Finite Element Method (FEM) Analyses

Prior to the actual construction of the proposed remedial works, finite element method (FEM) analyses on the proposed remedial solution were carried out to analyse the overall con-figuration in order to determine the performance of the pro-posed remedial solution. Figure 25 shows the model of RS Wall used in the FEM analyses. The Mohr Coulomb soil model was adopted in the analyses. In FEM modelling, both the spun pile and RC Raft were modelled as linear elastic ma-terials with properties of Young’s Modulus and Poisson’s ra-tio. The spun piles and the RC raft were modelled as beam elements in order to determine the shear force and bending moment for detailed design thereafter. A summary of the ma-terial properties is presented in Table 5.

Figure 20 Excavation of Backfill Material, Hack Base Slab and Install Spun Piles

Figure 21 Reinstatement of RS Wall and Partial Re-placement of Backfill Material using EPS

Figure 22 Detail ‘A’

Figure 25 Reinforced Soil Wall Model

RS Wall

RC Raft

Strip

Spun Piles G.W.L

Original Ground Profile

Hard Layer

10

Table 5: Material Properties for FEM Analyses

Name Type γsat

(kN/m3) ν Eref (kN/m2)

Granular Backfill

Drained 18 0.3 50,000

Existing Soil Drained 20 0.3 12,500

Hard Layer Drained 20 0.3 125,000

RS Wall Panel

Elastic - 0.2 -

RC Raft Elastic - 0.2 - Pile Elastic - 0.2 - Strip Elastic - - -

Name cref (kN/m2)

φ (°) EA (kN/m)

EI (kNm2/m)

Granular Backfill

1 36 - -

Existing Soil 3 33 - - Hard Layer 3 33 - -

RS Wall Panel

- - 3.64 E6 5,945

RC Raft - - 6.5 E6 33,900 Pile - - 1.65 E6 34,976

Strip - - 4.69 E4 -

The checking for punching shear, normal shear, maximum shear and bending moment on the RC raft indicates that a minimum thickness of 300mm slab with T16@125mm rein-forcing steel bars are required. The structural elements were designed according to BS8110: 1985.

7.2 Reinforced Soil Wall Design

In the remedial design, the RS wall has been designed with consideration of external and internal stability according to BS8006:1995. The coherent gravity method was adopted for internal stability check.

8 PROPOSED MONITORING PROGRAMME

A monitoring programme was recommended to monitor the long-term performance of the proposed remedial work. The proposed monitoring programme consisted of three (3) incli-nometers. However, one inclinometer was damaged during in-stallation. Therefore, only two (2) inclinometers were in-stalled. The locations and typical details of inclinometers are shown in Figures 26 and 27. Readings of the instruments were recommended to be taken fortnightly for the first two (2) months and once a month thereafter for eight (8) months. Subsequently, the monitoring was carried out once every four (4) months for a year. Figures 28 to 31 show that the inclinometer monitoring results for Inclinometers 1 and 2 respectively in comparison with the FEM analyses. Upon reviewing the inclinometer monitoring results, it was found that the lateral displacement of the incli-nometer showed a trend of stabilising readings and are within the acceptable limit as interpreted from the FEM analyses. Therefore, it can be concluded that the proposed remedial works are effective.

Figure 26 Location Of Inclinometers

Figure 27 Details of Inclinometers

Figure 28 Inclinometer Monitoring Results for Incli-nometer 1 (Major Axis)

Slope

R.E. Wall

B - B +

-16

-14

-12

-10

-8

-6

-4

-2

0-5 5 15 25 35 45

Displacement (mm)

Dep

th (m

)

9-Feb-0424-Feb-0412-Mar-0413-Apr-0417-May-0415-Jun-0416-Jul-0410-Jan-05 (Final)Design Limit

R.E. Wall Slope

Slope

R.E. Wall

B - B +

Design Limit

11

9 SUMMARY AND CONCLUSIONS

The investigation results of RS wall failure at the petrol sta-tion were presented. The wall bulging occurred progressively had caused the backfilled material to be washed away during rain and induced voids behind the RS wall. In an attempt to identify possible causes of failure, a comprehensive investiga-tion was carried out.

The conclusions and recommendations can be summarised as follows:

Slope Stability Analyses

• Local stability of the fill slope in front of the RS wall was not the causes of the bulging problems.

• Global stability of overturning and sliding of the RS wall were adequate, except for the global slip failure through the slope under the loading from the wall. Slope stability analyses indicate that the global slip failure under the loading from the RS wall was in-adequate if the RC piles had been displaced.

RS Wall Internal Stability Analysis

• According to BS8006:1995, all aspects of RS wall internal stability were adequate except for adher-ence. The wall strip reinforcements had inadequate FOS against adherence failure.

• There was insignificant increase in the acting tensile force in the presence of the groundwater table for 7.5m RS wall. However, the adherence was com-paratively lower with the presence of the groundwa-ter table than that in the absence of the groundwater table. As such, it is recommended to consider the ef-fects of the groundwater table (hydrostatic pressure) on the reinforced soil wall internal stability design.

Structural and Geotechnical Capacity Check of Piled Founda-tion

• Finite element method (FEM) analyses show sig-nificant movement of the 150mm x 150mm RC piles supporting the wall.

• The shear and bending moment resistances of 150mm x 150mm RC piles and the base slab are in-adequate.

Remedial Work Design

• The remedial works consist of reconstructed part of RS wall to be supported by 450mm diameter spun piles (filled with reinforced concrete plug) and par-tial replacement of existing backfill materials with lightweight Expanded Polystyrene (EPS).

• The monitoring results of the inclinometer show a trend of stabilising readings and are within the ac-ceptable limit as interpreted from the FEM analyses. The proposed remedial works are effective.

REFERENCES

British Standard Institution. 1995. BS8006:1995 British Stan-dard Code of Practice for Strengthened / Reinforced Soils and Other Fills. British Standard Institution. 1994. BS8002: 1994 British Code of Practice for Earth Retaining Structures The French Ministry of Transport. Reinforced Earth Struc-tures: Recommendations and Rules of the Art. Tschebotarioff G.P. 1973. Foundations, Retaining and Earth Structures, 2nd Edition. McGraw-Hill, New York. Tan, Yean-Chin & Khoo, Chee-Min. 2007. Effects of Hydro-static Pressure on Limit State Design of Reinforced Soil Wall Internal Stability, Proc. of 16th SEAGC, Kuala Lumpur (sub-mitted).

Figure 29 Inclinometer Monitoring Results for Incli-nometer 1 (Minor Axis)

Figure 30 Inclinometer Monitoring Results for Incli-nometer 2 (Major Axis)

Figure 31 Inclinometer Monitoring Results for Incli-nometer 2 (Minor Axis)

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